The importance of freshwater to estuaries
Productivity in natural systems refers to the amount of biomass produced by living organisms over a given period of time. Estuaries are one of the most productive ecosystems on earth, and freshwater inflows are the single most important determinant of productivity in estuaries. Freshwater inflows to estuaries are important because they maintain the salinity needed to sustain juvenile fish and invertebrates such as shrimp and crabs; provide nutrients and phytoplankton (floating algae, mostly microscopic) that form the base of the food web; flush pollutants from bays; sustain a variety of habitats from mangroves and salt marshes to seagrass beds to oyster bars; and signal fish to spawn or move to different habitat.
All of the rain that falls on the land eventually makes its way to a lake, river, stream, estuary or ocean. Some rain water flows slowly, in a thin layer called sheet flow across the land, and other rain water soaks into the ground where it travels underground. The land area that drains water to a common waterbody is referred to as a watershed. We all live in a watershed.
Watershed elevations and soil types determine surface and groundwater flows into estuaries. Estuaries are sensitive to the timing and amount of freshwater delivery, and different estuaries are adapted to different freshwater regimes. The mixing of fresh and salt water in estuaries varies with geology, hydrology, and human alterations such as dredging of new or deeper channels. Tides also play a large role and have a varied effect. Together, tides and winds help mix freshwater and saltwater together. A mixture of fresh and saltwater is commonly referred to as brackish and found throughout estuaries.
Estuaries are where most of our important Gulf species live before becoming adults. It is here that young fish, shrimp and crabs find food and hide from predators. In the upper reaches of estuaries, freshwater inflow transports sediments that settle along the bottom in quiet water areas. These sediments provide habitat for plants and burrowing animals such as worms and clams.
Contained in these transported sediments are tiny bits of detritus (organic material such as leaf litter) and nutrients, such as nitrogen and phosphorus that feed rooted plants, microscopic plankton (floating plants and animals) and other living things. The microscopic plankton are filtered from the water by oysters that build reefs. These reefs provide more food and shelter for fish and crabs.
Changes in freshwater inflow acts as a trigger telling many fish it’s time to move. Smalltooth sawfish, for example, are known to move up and down river systems as freshwater inflow changes in order to stay in their optimal salinity range. Most recreationally and commercially important fish species that rely on our estuaries also use salinity as a cue to spawn. As examples, snook move out to the passes to spawn mostly in the summer during seasonally high freshwater inflow and mullet in the fall and winter when freshwater inflow wanes.
When the rivers are flooded due to high inflows, some fish species, both juveniles and adults, will move into inundated floodplain areas. For example, each year, some snook travel way up into the Peace River floodplains after spawning. They remain upriver until early spring when the river dries and they are forced back down to the estuary. Inundated floodplains provide a buffet of organisms for savvy snook to feed on. The more flooded the floodplain, the fatter the snook are when they return from their winter feast.
This year, for us in southwest Florida, there is another reason to appreciate freshwater inflows. Red tide cannot grow in salinities less than 18 practical salinity units (psu) and prefers salinities higher than 25psu. Right now that’s like drawing a line from Cape Haze Point over to Burnt Store Marina. Everything north of that line in Charlotte Harbor has been too fresh for red tide this summer, making these areas an important refuge for many fish and wildlife species. The upper harbor has not been immune to the funk smell of decaying fish, invertebrates, sea turtles and marine mammals carried in by the tide, but the red tide organism Karenia brevis has stayed out.
Reduced freshwater to estuaries is a concern in many areas. In Florida Bay, where delivery of fresh water has been significantly altered, numerous negative effects have been noted, including reduced recruitment of pink shrimp, snook, and redfish and lower reproductive success of ospreys and great white herons. In Apalachicola Bay, reduced flows have had deleterious effects on the area’s oyster fishery.
Here we’ve also seen some reductions in freshwater flow. Kissingen Springs used to provide 20 million gallons of water per day to the Peace River. The spring, in upper Polk County was also a popular recreation spot; at least it was until it dried up in the 1950s.
Now it can certainly be said that too much freshwater, such as what we are seeing in the Caloosahatchee River estuary is harmful to an estuary; but so too is too little freshwater. And of course when I discuss the importance of nutrients, I am referring to the normal nutrient flow. Too many nutrients upset the natural balance of things and are never good!
How much, when AND at what quality freshwater flows into our estuaries are critical determinants of the functioning of these aquatic ecosystems.
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Poulakis, Gregg. 2018. Email communication.
Vargo, Gabriel A. 2009. A brief summary of the physiology and ecology of Karenia brevis Davis (G. Hansen and Moestrup comb. nov.) red tides on the West Florida Shelf and of hypotheses posed for their initiation, growth, maintenance, and termination, Harmful Algae 8: 573–584.
Reviewers: Gregg Poulakis, Ashley Smyth & Lisa Krimsky